Fix an incorrect spelling in 'Quickstart for writing on GitHub'

[word-wiz]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/quickstart-for-writing-on-github

What part(s) of the article would you like to see updated?

One small incorrect spelling, 'expandible', in section 'Adding a collapsed section'.

Additional information

No response

[welcome]

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[nguyenalex836]

@word-wiz Thank you for opening an issue and linking it to your PR! ✨